Near infrared spectroscopy (NIRS) is a well-established non-invasive technique which allows for the determination of tissue and blood analytes conditions based on spectrophotometric measurements in the visible and near-infrared regions of the spectrum of light. According to this technique, incident light penetrates the examined skin, and reflected and/or transmitted light is/are measured. In order to quantify any blood analyte, light of at least two different wavelengths is required. Optical plethysmography, pulse oximetry, and occlusion spectroscopy are the most prominent examples of usage of the NIR spectroscopy in medicine and physiological studies.
Visible or near infrared light is commonly used to track the optical manifestation of some time-dependent physiological processes. Such prolonged measurement of light response as a function of time can provide clinician with valuable information about time-dependent physiological processes.
For example, the measured light response of a natural heart beat pulsation is varied with each pulse. The signal is then measured at one point of the pulse wave and compared with the signal at another point. The difference between the values is due to arterial blood alone. In the pulse-oximetry, this phenomenon is utilized for the determination of oxy-hemoglobin saturation.
In the case of occlusion spectroscopy, the optical time-dependent signal is originated by light scattering changes associated with the red blood cells (RBC) aggregation process. In this case, the optical signal changes are utilized for the hemoglobin or glucose measurement.
Yet another known method enables to generate the required changes is the application of a periodic or non-periodic local pressure variation, resulting in blood volume fluctuations. These fluctuations are used to measure different blood parameters, like hemoglobin or glucose.
The major underlying assumption in the processing of all kind of the time-dependent signals is that the measured optical variation is originated solely by blood related components. In pulse oximetry, for example, it's commonly accepted that arterial blood volume changes are the only responsible factor staying behind the optical signal modulation. However, a more complex physical analysis shows that even if the only changes in the system are ascribed to the blood, the measured optical response of these changes is a convolution of absorption and scattering properties of blood and surrounding media. While carrying out any algorithmic modeling and signal processing procedure of these measured optical signals, the tissue related effects can not be disregarded. Therefore, the common denominator of all time-dependent signal related optical methods relies on the measurement of optical responses originated by the blood dynamics or hemorheological status changes.
It should be noted that the accuracy of time-dependent methods depends on the ability to identify the hemorheological component of the blood. For example, in the particular case of pulse-oximetry, the heart beats modulate the hemorheological status of circulating blood, resulting in the fluctuation of RBC velocity, which is associated with the shear forces changes. The variation of the hemorheological blood parameters enables to optically distinguish the pulse-related changes of the signal. Therefore, the decreased accuracy in the determination of hemorheological properties leads to a lower accuracy in the determination of the sought blood parameter. Among the blood parameters which can be derived from the hemorheological changes are hemoglobin oxygen saturation, carohyhemoglobin (percentage of HbCO out of total hemoglobin), hemoglobin blood concentration and/or glucose.
Moreover, the arterial blood pressure is another physiological parameter, which is commonly derived from the hemorheological related variations. The systolic blood pressure can be determined with assistance of inflating cuff which induces hemorheological variations artificially. When a pressure beyond the systolic pressure is applied, no pulsatile waveform appears at the down-flow. The diastolic point of the pressure is frequently measured by using Korotkoff's sounds. The source of these sounds is associated with abrupt changes in hemorheological properties of blood, occurring due to deflation of cuff from the systolic point. These hemorheological changes, in the vicinity of the diastolic point, result in a very typical pattern of sound, which can be detected by a stethoscope or by other acoustic device. However, the sound related method is very sensitive to different motion artifacts and therefore in automatic blood pressure devices, commonly used for the self-monitoring, the accuracy of blood pressure reading is impaired.